Fix invalid PHP constants.

Code was generating invalid PHP constants for enums.

For example,

const NEW='new';

To prevent all constants are now prefixed with C_

For example,

const C_NEW='new';

In addition the code now generates phpunit tests for enum classes. This
should have been done from the start as running the tests would have
found this issue sooner.
Merge branch 'enums' into develop
Merge branch 'develop'